sol.

Certainly inherited from the Proto-Indo-European word for "sun," which might have had a form like *sóh₂wl̥ (see there for alternative reconstructions). The development of the Latin form and the exact reconstruction of the PIE word is disputed. The only attested cognate within the Italic branch is Oscan 𐌔𐌖𐌋𐌄𐌉𐌔 (suleis) (genitive singular), which may represent underlying *sōl or *sūl. In the former case, the term would be identical to the Latin word and would then support reconstructing the vowel *ō at the Proto-Italic stage. Poccetti 2016, following a suggestion by V. Martzlof, indicates that both words could be derived from Proto-Italic *swōl, from Proto-Indo-European *sh₂wōl. If, however, the Oscan form is taken as *sūl, then it may reflect Proto-Indo-European *sh₂ul and the Latin /ō/ would presumably reflect an independent development. De Vaan and Sihler have both proposed theories regarding the origin of this Latin word and the reconstruction of the PIE term, though they did so prior to the discovery of the Oscan evidence: * According to De Vaan, perhaps inherited from Proto-Indo-European *séh₂ul ~ *sh₂wéns. However, per De Vaan, such a PIE term is expected to yield Latin **saul and a zero-grade sequence **sh₂ul should have produced **sūl. To remedy this issue, Frederick Kortlandt and Robert Beekes have suggested that the unusual vocalism is the result of a particular development conditioned by the -l. * Sihler suggests that a pre-form of the shape *suh₂el yielded Proto-Italic *suwal, whence Old Latin *suwol and later *suol, which perhaps then contracted to sōl. Sihler compares this development to the shift of *swo- to so- as in soror.